A drill I really like for sparring (especially against very good fighters) is what my coach calls "Throw 4"
In this excersise, your opponent throws any 4 punches he wants at you, then when he's done, you throw 4 back. When I first started boxing, I found this really helpful. It's good because:
1) You know you won't take a prolonged ass-whoopin' even if someone is a lot better then you. You know you'll only be hit 4 times at the most at a time instead of relentless combos.
2) You can see how your opponent throws his punches, what he hits you with, and when you throw, what you hit him with, what he avoids, and he avoids them.
3) It works wonders when working on throwing in combinations (see what you hit him with, what misses).
Of course this only works for a while, because as you get more advanced you'll want to add counterpunching etc. but it's good for beginners.